The present invention relates to a bipolar transistor circuit in general, or more in particular to a drive circuit for a bipolar transistor suitable for integrated circuits that include a bipolar transistor and a MOSFET that can be fabricated on the same substrate.
Conventional drive systems for bipolar transistors using a field effect transistor (MOSFET) in integrated circuits are classified into the types shown in FIG. lA to FIG. 1E relating to an NPN bipolar transistor (hereinafter sometimes referred to as NPN or NPN transistor). FIGS. 2A to 2E show a similar classification relating to a PNP bipolar transistor (hereinafter sometimes referred to as a PNP or PNP transistor). In addition to these systems, there may be more complicated systems with combined series and parallel connections. In these classifications, those methods which are different only in the method of drawing the base current are considered to belong to the same category.
In Figs. 1A to 1E and 2A to 2E, reference numerals 200 to 206 designate signal input terminals, numerals 207 to 211 bipolar transistors which driven are, numerals 212 to 218 MOSFETs for driving them and numerals 100 and 101, negative and positive power supply terminals, respectively.
The feature of the combined circuits shown in Figs. 1A to 1D and FIGS. 2A to 2D lies in that a bipolar transistor is turned on by short-circuiting the collector and base thereof by means of a MOSFET, and turned off by opening the collector-base circuit thereof.
The systems shown in Figs. 1E and 2E, on the other hand, in which the gate of a MOSFET is connected to the collector of a bipolar transistor, is not applicable to a circuit having a plurality of input terminals. The present invention will therefore be explained below in comparison with the drive systems of the former type.
Systems of this type are disclosed, for example, in JP-A-No. 61-53828, U.S. Pat. No. 354135, JP-A-No. 59-19435, JP-A-No. 60-16022, JP-A-No. 60-51326, JP-A-No. 59-25424, JP-A-No. 61-93655 and JP-A-No. 61-289723.
Explanation will now be made about a problem posed by the prior art, that is, a method in which the collector and base of a bipolar transistor are short-circuited by a MOSFET.
Generally, if the switching speed of a bipolar transistor is to be increased, it is necessary to supply an increased amount of base current. In the conventional system of driving a bipolar transistor by a MOSFET, however, if a sufficiently large capacity of current supply of a MOSFET, that is, the conductance of a MOSFET turned on, is not obtained for some reasons or other, the switching rate of the bipolar transistor is determined undesirably by the capacity of the MOSFET, and the intended performance is not achieved. With the reduction in the gate length, for instance, the withstanding voltage of the MOSFET is decreased, and therefore if a fine MOSFET of high performance is to be used, it may be necessary to reduce the voltage impressed on the MOSFET. In such a case, however, a smaller voltage applied to the gate of the MOSFET would reduce the conductance of the MOSFET in its on state, thereby making it impossible to attain a high performance which otherwise could be attained with reduction in size.